CA1054263A

CA1054263A - Axial tomography

Info

Publication number: CA1054263A
Application number: CA278,498A
Authority: CA
Inventors: Keith A. Brueckner; John H. Lewis
Original assignee: Xonics Inc
Current assignee: Xonics Inc
Priority date: 1976-09-10
Filing date: 1977-05-16
Publication date: 1979-05-08
Also published as: GB1540345A; FR2364019A1; DE2740367A1; JPS5335397A; NL7705995A; IT1090055B; US4072289A; BE855760A

Abstract

ABSTRACT OF THE DISCLOSURE
Analog method and apparatus for producing an axial tomographic image of an object, typically the human body, using a detector for receiving radiation along a plurality of sets of paths, with the sets of paths overlapping each other, and providing a plurality of sets of detector output signals.
Prior art devices exist for producing the detector output signals, which signals are then manipulated in a digital computer to produce the desired image. In one embodiment of the present invention, the detector output signals are summed in a storage tube and the stored signals are read in an inversely proportional relation, providing another signal for storage in another storage tube which is then read with a Laplacian re-lation to give a video signal for display and/or recording.
Optical and electrostatic storage systems are also disclosed.

Description

- 'I 1054Z~3 1 ¦ : BACKGROUND OF THE INVE~ITION

2 ¦ This invention rela~es to axial tomography, sometimes

3 referred to as cross-sectional X-ray. A general discussion of

4 ¦ the subject appears in the article "Image Reconstruction frGm Projections" by Richard Gordon, et al appearing in the October 6 ¦ 1975 issue of Scientific American pages 56-68.
7 ¦ In a typical axial tomographic system, a detector 8 ¦ receives radiation from an X-ray source along a plurality of 9 sets of paths with at least some of the paths of a set traversing 10 ¦ the object of interest, and with the sets of paths overlapping 11 ¦ each other providing a plurality of sets of radiation detector 12 ¦ output signals. These detector signals are then utilized in a 13 complex computer operation to produce the desired image. This 14 type of system is described in ~ritish Patent 1,283,915, in the article "Theory of Image Reconstruction in Computed Tomography"
16 by Rodney A. Brooks, et al appearing in Radiology 117: 561-572, 17 December 1975, and in the article "Computerized Transaxial X-ray 18 Tomography of the Human Body" by R. S. Ledley, et al appearing in 19 Science, 18 October 1974 Vol. 186 Number 4160.
These prior art systems utilizing a computer for data 21 handling are relatively expensive because of the computer ~2 capacity required. Also, a significant amount of time is re-23 quired in performing the computations for an image.
24 The prior art systems require digitizing of the detector output signals with a resultant loss in resolution. In order to 26 improve resolution, one would have to increase the X-ray dosage 27 which is undesirable in most instances. A typical prior art compu 28 terized axial tomography system will provide a display matrix of ~ . .
.

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1 180 bits by 180 bits for a brain section. In comparison, the 2 analog system of the present invention utilizing a standard 3 T-V monitor as the disolay can provide 1000 bits b~ 1000 bits 4 ¦for a brain section.

5 ¦ In addition, the prior art systems produce images which

6 lare degraded in quality if the object being radiog-aphed is allow-

7 ¦ed to move, even in the slightest manner, during the sequential

8 ¦detection (scanning) process. Such motion causes the digital

9 ¦i~age reconstruction algorithms to produce artifacts which are difficult and time consuming to correct within the co~puter. In 11 comparison, the analog system of the present invention does not 12 ¦suffer the same degree of difficulty, since such motion-induced 13 artifacts are not generated during the analog image reconstruction 14 Drocess.
Accordingly it is an object of the present invention to 16 provide a new and improved method and apparatus for axial tomo-17 graphy which may use the conventional radiation scanning config-18 urations while improving resolution and image quality without 19 equiring increases in X-ray dosage. A particular object is to 20 provide such a method and apparatus which operates in an analog 21 method rather than in a digital method, and an apparatus which is Z less expensive than the prior art systems.
23 An additional object of the present invention is to pro-24 ¦vide an analog method of image reconstruction which is applicable 2~ ¦to tomographic images generally, whether produced by gamma ray, 26 X-ray, or ul~rasonic radiation. The detected quality of radiation 27 can be either the transmitted X-ray beam which is a measurement of 28 attenuation along a ray, as is the case of the prior art systems, 30 or an emitted beam which is a measuremeat of source strength along æ .
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1 a collimated ray, as is the case in nuclear medicine procedures.
2 ¦Ultrasonic radiation can be measured either as an intensity trans-3 mitted signal or as a temporal (time of flight) signa~. The 4 ¦integrated velocity of the ultrasonic beam through the imaged D ¦object is a measure of the material dispersion as opposed to 6 attenuation and constitutes additional information which can be imaged.

9 S ~ ~ARY OF THE I~IVE~TION

10 ¦ The apparatus of the present invention may utilize the

11 ¦conventional X-ray or ultrasonic source and detector and scanning

12 ¦mechanism for producing the plurality of sets of radiation detecto

13 ¦output signals. The apparatus further includes means for storing

14 ¦the detector output signals in analog form with the signals of

15 one set overlying the signals of another set so that signals re-

16 sulting from radiation through a zone of the object being examined

17 are summed at a corresponding zone in the storage device, which

18 typically is an electronic storage tube. These summed signals

19 re read from the storage device with a radially inversely pro-ortional reader producing a second signal for storage, again 21 typically in an electronic storage tube. These signals stored in æ the second storage device are read with a Laplacian relation, 23 ith the resultant signal being a video signal suitable for 24 onnection to a TV monitor for display of the sectional image.
2~ This display may be photographed for a permanènt record and 26 lternatively, the video signal may be recorded for a permanent 27 ecord. In alternative embodiments, optical film systems and S0 lectroseaeic .yeeem: are ~ lized.

33l . . ( I' 1 ~0542~3 ' I . - , 1 ¦ The method of the present invention includes storing 2 ¦the detector output signals in analog form wLth signals of one 3 ¦set overlying signals of another set summing the signals result-4 ¦ing f~om radiation through a zone of the object being examined at a corresponding storage zone, as by writing the signals in an electronic storage tube or exposing a photographic film or 7 ¦generating charges on a dielectric. The summed signals are read 8 in a radially inversely proportional relatianship to provide a 9 second signal which in turn is stored. The stored second signal 10 ¦is read with a Laplacian relation to provide an output which is 11 a video signal corresponding to the desired sectional image.

13 BRIEF DESCP~IPTION OF THE DRAWINGS
14 ¦ Fig. 1 diagrammatically illustrates one known form of axial tomography scanning utilizing a pencil beam;
16 Fig. 2 is a block diagram of an apparatus for using 17 the detector output and incorporating the presently preferred 18 embodiment of the invention;
19 Fig. 3 is a graph illustrating a preferred reading eam electron flux density for the first storage tube of the 21 apparatus of Fig. 2;
22 Fig. 4 is a diagram similar to that of Fig. 1 illus-a3 trating a fan beam configuration;
Fig. 4A is a diagram illustrating a detector array with collimation, 26 Fig. 4B is a diagram illustrating operation with an 27 ultrasonic source and detector;
28 Fig. 5 is a block diagram of an optical apparatus 29 incorporating an alternative embodiment of the invention; and Fig. 6 is a block diagram of an electrostatic apparatus 32 incorporating an alternative embodiment of the invention.

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. 1C)54263 1 DESCRIPTION OF THE PP~EFERRED E~3ODIMENTS
2 Fig. l illustrates a conventional scan configuration 3 used in axial tomography. An X-ra~ source 10 produces a pencil 4 beam directed to a radiation detector 11 along a path through the object (object path) 12. The X-ray source and detector are 6 scanned across the object 13 providing radiation intensity measurements along a set of parallel paths, typically 100 paths.
8 The scan mechanism is then rotated relative to the object, typically one degree, and a second scan is performed providing a second set of parallel paths. Three sets of such paths, scan 1, 11 scan 2 and scan 3, are shown in Fig. 1. In the conventional axial 12 tomography system, the dete~tor output signals are digitized for, 13 use in computing the cross sectional image of the object 13.
14 In the presently preferred embodiment of the invention illustrated in Fig. 2, the detector 11 is scanned across the 16 object by a scanner 15, with the detector output on signal line 17 16 being written into an electronic storage tube 17. The detector 18 output signal for object path 12 is stored as a line in the 19 storage tube, with the set of paths of a scan providing a set of lines at the storage tube. The detector output signal for the 21 set of paths for the next scan are written as lines at the 22 storage tube over the lines of the previous scan. Thus the 23 detector output signals for any point in the object 13 are 24 summed at a corresponding point at the storage tube 17. For example, the detector output signals for radiation passing 26 through the point 20 of the object 13 are summed at the point P7 21 of the storage trbe 17.
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'., 1 The signals now stored in the storage tube 17 are 2 read by a scanner 24, typically using a conventional raster 8 scan, with the output being stored in a second storage tube 25, 4 with the signal at point 21' in the tube 25 corresponding to ~ the signal at point 21 in the tube 17. Similarly, the signal 6 at point 22' in the tube 25 corresponds to the signal at 7 point 22 in the tube 17.
8 The signals at the storage tube 25 are read by another 9 scanner 26 providing an output on signal line 27 in the form of a video signal suitable for display at a cathode ray tube 28 provid-11 ing the desired reconstructed tomographic image of the object 13.
12 The directions of the image scans correspond to the 13 geometry of theradiation source and detector, usually on an 14 altered scale. The complete tomographic image is formed by ~ the accumulation of the images for the co~plete rotation of the 16 detector system. To avoid edge effects in the final processed 17 image, the recorded tomographic image desirably should be 1 approximately 50% to 100% larger in linear dimension than the 19 expected size of the imaged object.
ao The tomographic image T at point (r) is closely equal 21 to æ
24 T(r) = ¦ ~ dr' (1) with the integral carried over the two-dimensional object plane.
26 The visual object O(r) can be recovered from the tomographic 2q image by the geometric process 2g 3(r) = v2 S(r) (2) S(r) ~ (3) ~a 7 .
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. 1 1054Z6 ~ j 1 ¦The integral over Ttr') giving S(r) can be performed by a 2 ¦detector ~Jith response inversely proportional to distance from 3 the scan point, which integrates the image intensity, The 4 ¦derivatve of S(r) required to form the image is determined by 6 signal subtraction.
. a2 S(r) = constant x [S(x + ~,y) + S(x - ~,y) +
8 1 . S(x, y + Q) + S(x, y - ~) - 4 S(x, y)] (4) .
. 9 .
10 ¦Thus the reconstructed object signal at x, y is formed from the 11 5 measurements at the points x + ~, y; x, y + a; x, y. This can 12 ¦be done by sequential shift of the integrator or by digital 13 recording of the output for subsequent subtraction or by pulsed ~4 or focus modulated e-beam non-destructive reading of S(r) re-construction.
16 The electron beam of the scanner 24 is tailored to 17 provide the desired inversely proportional relationship l/r.
18 The desired electron flux density in the beam is illustrated 19 in Fig, 3, with the electron flux density decreasing as the in-erse of the distance from the center of the beam. This may be 21 achieved using conventional techniques, such as fixed focus con-æ trol shaped electron beam, or scanning bet~een a focussed and a 23 defocussed condition, or by means of a spiral pencil beam.
. 24 In the preferred embodiment illustrated, the Laplacian 2$ v2 may be obtained by utilizing a pulsed pencil beam. By way of 26 example, for reading at the point 31, the beam may be pulsed four 27 times at point 31 and one time at each of four surrounding points, . - ~ith the readings for the signals at point 31 subtracted fro- the :' 33o -- ~

. 32 1 1054 ~3 l~
1 ¦readings for the four surrounding points. Additionally there2 lexists standard techniques for obtaining "Laplacian Enhancement"
3 by modulating the focus of the scanning electron beam about the 4 ¦point of interest. The spiral pencil beam may be a pencil elec-5 ¦tron beam which is projected in a spiral of uniform angular 6 ¦velocity about the scan point providing the desired integrated 7 reading which is inversely proportional to the radial distance f the beam from the scan point.
9 ¦ The present invention is not limited to the specific 10 ¦embodiment shown in Figs. 1 and 2 and a variety of components 11 may be utilized. By way of example, an X-ray source providing 12 a fan beam may be utilized in place of the pencil beam. Also, 13 an external X-ray source is not required and the system may 14 be utilized with an object having one or more radiation sources 15 within the object, such as is used in nuclear medicine techni~ues.
16 For nuclear medicine imaging, the detector should in-17 corporate a collimating grid which limits the aperture and the 18 emitted radiation beam. The detector may take various forms 19 including scintillators, a strip of film, a continuous scintil-lator backed by a photocell array, a photo conductorlphoto 21 emitter sandwich such as Csl/SbCs, and an electron radiographic æ real time recorder. The radiation source and detector may be 23 scanned in synchronism or one may be scanned with the other 24 stationary. One alternative configuration is illustrated in Fig.
2~ 4, with radiation source 10' providing a fan beam of radiation to 26 a detector ll'. This arrangement provides a set of converging 27 radiation paths rather than the set of parallel paths of the 28 configuration of Fig. 1. After exposure and recording of detector 29 utput signals along each path of the set, the source and detector 30 are rotated relative to the object 13 to provide another scan, 31 illustrated as scan 2 in Fig. 4. Typically, scans will be made 32 t one degree intervals through 180 degrees of rotation. When ~L054 ~
1 ¦ utilizing radioactive sources within the body under examination, ¦ collima~ion should be provided for the detector array, such as 3 ¦ shot~n in Fig. 4A, where metal sheets 32 are positioned in front 4 ¦ of the detectors of the array 11' to define a path to each detector element. An arcuate detector array as shown in Fig. 4 6 ¦ can be used to provide converging paths. Alternatively a linear 7 ¦ array can be used to provide parallel paths. In another 8 ¦ alternative, a collimated single detector may be scanned across ¦ the body either along a straight path or an arcuate path as 10 ¦ desired.
11 l~hile the preceeding discussion has referred to X-ray 12 ¦ and gamma ray sources and detectors, the present invention is i3 ¦ equally applicable to systems using ultrasonic sources and 14 ¦ detectors. The attenuation of the ultrasonic beam or the re-duction in velocity of the ultrasonic beam can be measured to 16 provide the detector output signals. Both techniques are con-17 ventional.
18 Ultrasonic generators are considerably less expensive 19 than ~-ray sources, and an array 10' of ultrasonic signal gen-erators may be used as shown in Fig. 4B, with the generators 21 driven in sequence to provide the sets of paths through the 22 body 13 to the detector array 11'. With this configuration there 23 is no requirement for motion of the source or detectors. In 24 ultrasonic systems, the source, detector and object may be immerse 1 ~ ¦ in a liquid medium 33 for roved energy covpling.

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~0542~i3 1 Several of these alternatives are illustra~ed in 2 the embodiment of Fig. 5. A photographic film detector is 3 ¦ utilized, with each scan of the object being recorded as a 4 line 35 on the film 36. After development, the film is read 5 ¦ by a densitometer 37 driven by a scanner 38 to provide the 6 ¦ detector output signals on signal line 39. Each line 35 corresponc s 7 ¦ to a set of paths, with each point being read along the line 8 ¦ providing the output signal for a single path. The detector 9 ¦ output signals atsignal line 39 may be stored in a storage tube, 10 ¦ such as the tube 17 of Fig. 2 However in the embodiment 11 illustrated, the output signals are stored on another photographic 12 ¦ film 42, with a line on the film 42 for each point on the 13 ¦ film 36, producing the overlying set of lines. The film 42 14 ¦ may be exposed by a lightspot source 43 ~hich us scanned across 15 ¦ the film by a scanner 44, with the light intensity varying as 16 a function of the detector output signals on thesignal line 39.
17 By way of example, points 47 and 48 of line 35 on 18 ¦ film 36 will be used to produce lines 47 and 48 on the film 42.
19 Similarly, points 50 and 51 of line 52 of film 36 will be used to produce lines 50 and 51 on the film 42. The signals stored 21 on the film 42 correspond to the signals stored on the storage 22 tube 17, with the summing being produced by the overlapping of 23 ¦ the exposures.
24 The film 42 is developed and then may be read by 25 ¦ another densitometer 55 driven by a scanner 56 which reads the 26 summed signals along a set of paths, typically a conventional 8D raster scan. The desired n~ersely propor~ion I rel-tion_hip 81 `

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0542~3 il ~ay be obtained by using a filter 57 in the reader, with the 2l filter having a transmission characteristic of l/r, where r is 3 ¦ zero at the optical axis of the reader 55.
¦ The output of the reader 55 on signal line 5~ is S(r), 5 ¦ corresponding to the output on signal line 59 of the embodiment 6¦ of Fig. 2.
7 ¦ The signals on line 58 may be stored in the second 8 storage tube 25 and then handled in the same manner as described g ¦ in conjunction with the embodiment of Fig. 2. Alternatively, the signals may be connected to a mini-computer 60 where they 11 ¦ are digitized, stored in a memory, and used in computing the 12 ¦~aplacian as shown in equation (4). The co~puted Laplacian is 13 ¦the desired image ready for storage and/or display at 61.
14 ~ypical storage devices include video tapes and video discs.
1~ ~ypical display devices include cathode ray tubes, photographic 16 ¦ilm, and electrostatic printers.
17 ¦ An electrostatic system is utilized in the embodiment 18 iLlustrated in Fig. 6, with the output signals from the detector 19 ¦ll being used to drive an electrostatic charge generator 65 such

20 ¦as an electron beam which is scanned over a dielectric sheet 66

21 ¦by a scanner 67 producing the overlying sets of paths correspond-

22 ¦ing to the pattern in the storage tube 17 of Fig. 2 and the film

23 ¦42 of Fig. 5, producing the desired summed signals.
2~ ¦ The charge pattern on the dielectric sheet 66 may be 2~ ¦read by a capacitor type charge reader 70 ~hich is driven by a 26 scanner 71, typically in a raster scan. The dielectric sheet 66 27 ¦~ay be placed on a metal plate 67 which serves as one plate of 29 I capacitor, wi~h the o~her late 68 carried ty the reader and ~ 312 ~ -12-I .
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''`. .' 1 having the plate 68 physically shaped to provide the inversely 2 ¦¦ proportional relationship. The charge reader output on signal 31 line 72 may be handled in the same manner as the reader output 4~ on the signal line 58 shown in Fig. 5.
Although exemplary embodiments of the invention have 6 been disclosed and discussed, it will be understood that the 7 embodiments disclosed may be subjected to various changes, 8 modifications and substitutions without necessarily departing 9 from the spirit of the invention.
The present invention provides high resolution and 11 ¦improved image quality with relatively low radiation dosage 12 ¦while elininating the requirement for digital storage of the 13 ¦detector-output signals and the expensive data processing both 14 lin terms of time and equipment required in the prior art systems ' ~tor handling the digll:ze ~e~ector outpUt signals.

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Claims

WE CLAIM:

1. In an apparatus for producing an axial tomographic image of an object with a detector for receiving radiation along a plurality of sets of paths, with said sets of paths overlapping each other, providing a plurality of sets of radiation detector output signals, the improvement including in combination:
first means for storing said detector output signals in analog form at a surface with signals of one set overlying signals of another set whereby the signals resulting from radiation through a zone of the object are summed at a corresponding zone at said surface producing a first stored signal;
first means for reading said first stored signal, including means for scanning across said surface and means for producing second signals, with a second signal for a scan point being a function of the magnitude of the first stored signal at the scan point and varying substantially inversely proportional to distance from the scan point;
second means for storing said second signals producing second stored signals; and second means for reading said second stored signals at each of a plurality of points of said second storage means and including means for producing a video signal for each point, with a video signal being substantially the Laplacian of the second stored signal for the point, and with the video signal corresponding to the desired image.

2. An apparatus as defined in claim 1 wherein said first means for storing includes an electronic storage tube.

3. An apparatus as defined in claim 2 wherein said first means for reading includes an electron beam of circular cross-section with the intensity of electron flux varying substantially inversely proportional to the distance from the center of the beam, with said beam being scanned across said surface reading said first stored signal.

4. An apparatus as defined in claim 2 wherein said second means for storing includes an electronic storage tube.

5. An apparatus as defined in claim 4 wherein said second means for reading includes a pulsed electron beam of substantially uniform electron flux density.

6. An apparatus as defined in claim 5 wherein said Laplacian is produced by summing the second stored signals for four points about the point of interest and four times the negative of the second stored signal at the point of interest.

7. An apparatus as defined in claim 1 wherein said second means for storing includes an electronic storage tube.

8. An apparatus as defined in claim 7 wherein said second means for reading includes a pulsed electron beam of substantially uniform electron flux density.

9. An apparatus as defined in claim 8 wherein said Laplacian is produced by summing the second stored signals for four points about the point of interest and four times the negative of the second stored signal at the point of interest.

10. An apparatus as defined in claim 1 wherein said first means for storing includes a photographic film and a radiation source for exposing said film as a function of said detector output signals.

11. An apparatus as defined in claim 10 wherein said first means for reading includes a film density reader.

12. An apparatus as defined in claim 11 wherein said film density reader includes a filter with a center and a transmission characteristic varying substantially inversely proportional to distance from said center.

13. An apparatus as defined in claim 11 wherein said second means for storing includes an electronic storage tube.

14. An apparatus as defined in claim 11 wherein said second means for storing includes a computer memory.

15. An apparatus as defined in claim 14 wherein said second-means for reading includes a computer which cal-culates said Laplacian using the second stored signals from said memory.

16. An apparatus as defined in claim 1 wherein said second means for storing includes a computer memory.

17. An apparatus as defined in claim 16 wherein said second means for reading includes a computer which cal-culates said Laplacian using the second stored signals from said memory.

18. An apparatus as defined in claim 1 wherein said first means for storing includes a dielectric sheet and an electrostatic charge source for charging said sheet as a function of said detector output signals.

19. An apparatus as defined in claim 18 wherein said first means for reading includes an electrostatic charge reader.

20. An apparatus as defined in claim 19 wherein said electrostatic charge reader is a capacitive type reader with a capacitor plate having a center and shaped to provide an output varying substantially inversely proportional to distance from said center.

21. An apparatus as defined in claim 2 wherein said first means for reading includes a pencil electron beam which is projected in a spiral of uniform angular velocity about the scan point to provide an integrated reading substantially inversely proportional to the radial distance from the scan point.

22. An apparatus as defined in claim 2 wherein said first means for reading includes an electron beam and means for modulating the focus of said beam to vary the intensity of electron flux substantially inversely proportional to the distance from the center of the beam, with said beam being scanned across said surface reading said first stored signal.

23. An apparatus as defined in claim 4 wherein said second means for reading includes an electron beam and means for modulating the focus of said beam to vary the electron flux density and produce said Laplacian of said second stored signal.

24. A process for producing an axial tomographic image of an object utilizing the output of a detector which receives radiation along a plurality of sets of paths, with the sets of paths overlapping each other, producing a plur-ality of sets of radiation detector output signals, including the steps of:
storing the detector output signals in analog form with signals of one set overlying signals of another set summing the signals resulting from radiation through a zone of the object at a corresponding storage zone producing a first stored signal;
reading the first stored signal by scanning across the storage zones and producing second signals, with a second signal for a scan point being a function of the magnitude of the first stored signal at the scan point and varying sub-stantially inversely proportional to distance from the scan point;

storing the second signal; and producing a electronic signal for each of a plurality of the second signals, with a electronic signal being substantial-ly the Laplacian of the second signal and with the electronic signal corresponding to the desired image.

25. The process as defined in claim 24 in which the detector output signals are stored electronically in a storage tube.

26. The process as defined in claim 25 which the inversely proportional relation is obtained by controlling a scanning electron beam.

27. The process as defined in claim 24 in which the detector output signals are stored optically on a film.

28. The process as defined in claim 27 in which the inversely proportional relation is obtained by varying the in-tensity of a scanning illumination beam.

29. The process as defined in claim 24 in which the detector output signals are stored electrostaticly on a di-electric.

30. The process as defined in claim 29 in which the inversely proportional relation is obtained by varying the shape of a scanning capacitor plate.

31. The process as defined in claim 24 in which the Laplacian relation is obtained by computation of digitally stored second signals.

32. The process as defined in claim 24 in which the second signals are stored electronically in a storage tube.

33. The process as defined in claim 32 in which the Laplacian relation is obtained by controlling a pulsed uniform density scanning electron beam.